U.S. patent number 7,234,360 [Application Number 10/405,934] was granted by the patent office on 2007-06-26 for tmr sensor.
This patent grant is currently assigned to Infineon Technologies AG, Stifting Caesar. Invention is credited to Markus Lohndorf, Alfred Ludwig, Eckhard Quandt, Manfred Ruhrig, Joachim Wecker.
United States Patent |
7,234,360 |
Quandt , et al. |
June 26, 2007 |
TMR sensor
Abstract
A sensor for measuring mechanical changes in length, in
particular a compressive and/or tensile stress sensor, includes a
sandwich system with two flat and superposed electrodes separated
from each other by a tunnel element (tunnel barrier), in particular
an oxide barrier, a current being set up between the electrodes and
through the tunnel barrier, one electrode consisting of a
magnetostrictive layer 3 which responds to elongation, and wherein
the contributions of the anisotropies caused by mechanical tension
are larger than those from the intrinsic anisotropies, relative
changes in system resistance .DELTA.R/R larger than 10% at room
temperature being attained during elongation.
Inventors: |
Quandt; Eckhard (Bonn,
DE), Lohndorf; Markus (Bonn, DE), Ludwig;
Alfred (Bonn, DE), Ruhrig; Manfred (Eckental,
DE), Wecker; Joachim (Rottenbach, DE) |
Assignee: |
Stifting Caesar (Bonn,
DE)
Infineon Technologies AG (Munich, DE)
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Family
ID: |
28684752 |
Appl.
No.: |
10/405,934 |
Filed: |
April 3, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040050172 A1 |
Mar 18, 2004 |
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Foreign Application Priority Data
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Apr 4, 2002 [DE] |
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102 14 946 |
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Current U.S.
Class: |
73/779 |
Current CPC
Class: |
G01L
1/125 (20130101) |
Current International
Class: |
G01B
7/16 (20060101); G01L 1/00 (20060101) |
Field of
Search: |
;73/779,763,728,754
;428/692,611,332 ;360/323,324.12,324,324.2 ;257/421,422
;365/170,171 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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198 30 343 |
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Apr 2000 |
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DE |
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198 36 567 |
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Dec 2000 |
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DE |
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199 49 714 |
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Apr 2001 |
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DE |
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100 09 944 |
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Sep 2001 |
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DE |
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100 28 640 |
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Dec 2001 |
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DE |
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Other References
"Neel `orange-peel` coupling in magnetic tunneling junction
devices", by B.D. Schrag et al, Applied Physics Letters, vol. 77,
No. 15, Oct. 9, 2000. cited by other .
"Layered Magnetic Structures: Evidence for Antiferromagnetic
Coupling of Fe Layers across Cr Interlayers", by P. Grunberg et al,
Physical Review Letters, vol. 57, No. 19, Nov. 10, 1986. cited by
other.
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Primary Examiner: Cygan; Michael
Assistant Examiner: Davis; O.
Attorney, Agent or Firm: Lowe Hauptman & Berner
Claims
The invention claimed is:
1. A sensor for measuring mechanical changes in length, comprising
a sandwich system with two flat and superposed electrodes separated
by a tunnel barrier, an electric current being set up between the
electrodes and through the tunnel barrier, wherein a first one of
the electrodes includes a highly magnetostrictive layer responding
to elongation, having signal contributions due to anisotropies
caused by mechanical tension being larger than those due to
intrinsic anisotropies, and providing relative changes in system
resistance .DELTA.R/R of more than 10% upon elongation at room
temperature; and the highly magnetostrictive layer includes an
alloy containing CoFe.
2. The sensor as claimed in claim 1, wherein a second one of the
electrodes includes a magnetic layer or a system of layers as a
reference layer, and wherein the reference layer has signal
contributions due to anisotropies caused by mechanical tension
being less than those due to intrinsic anisotropies.
3. The sensor as claimed in claim 1, wherein the relative changes
in system resistance .DELTA.R/R are between 20 and 50%.
4. The sensor as claimed in claim 1, wherein the highly
magnetostrictive layer includes a soft magnetic material exhibiting
large spin polarization.
5. The sensor as claimed in claim 1, wherein the thickness of the
alloy containing CoFe is less than 5 nm and said alloy is deposited
on a layer made of an anti-ferromagnetic material.
6. The sensor as claimed in claim 2, wherein the reference layer is
coupled by exchange coupling with a layer made of a natural
anti-ferromagnetic material, whereby unidirectional anisotropy is
created in the reference layer.
7. The sensor as claimed in claim 1, wherein the sandwich system
fitted with the three superposed layers is covered further wit
additional layers for the purpose of compensating
interferences.
8. The sensor as claimed in claim 1, wherein the tunnel barrier is
made by sputtering and exhibits a thickness of less than 50 nm.
9. The sensor as claimed in claim 8, wherein the tunnel barrier is
a layer of aluminum oxide oxidized by plasma oxidation and exhibits
a thickness of less than 5 nm.
10. The sensor as claimed in claim 1, wherein the layers of the
sandwich system are approximately rectangular and have all sides
less than 100 microns.
11. The sensor as claimed in claim 1, wherein the first electrode
is a magnetically hard layer exhibiting uni-axial anisotropy.
12. The sensor as claimed in claim 1, wherein the highly
magnetostrictive layer is composed of several single layers.
13. The sensor as claimed in claim 1, wherein the tunnel barrier is
made by sputtering and exhibits a thickness of less than 20 nm.
14. The sensor as claimed in claim 8, wherein the tunnel barrier is
a layer of aluminum oxide oxidized by plasma oxidation and exhibits
a thickness of less than 1.5 nm.
15. A sensor for measuring mechanical changes in length, comprising
a multilayer structure comprising a plurality of layers stacked one
upon another in a thickness direction of said structure; said
layers comprising upper and lower layers which are electrodes and
an intermediate layer sandwiched between said upper and lower
layers; said intermediate layer being a tunnel barrier which
exhibits the tunnel magneto-resistance (TMR) effect, and defines,
together with said electrodes, a tunnel current path tat extends
from the upper layer, through an entire thickness of the
intermediate layer and to the lower layer; and wherein said upper
layer includes a highly magnetostrictive material that responds to
stress applied thereto; and the highly magnetostrictive layer
includes a CoFe alloy.
16. The sensor as claimed in claim 15, wherein a material of the
lower layer is magnetically harder than the highly magnetostrictive
material of the upper layer.
17. The sensor as claimed in claim 16, wherein the magnetically
harder material of the lower layer exhibits uni-axial anisotropy
pointing anti-parallel to the orientation of the magnetically
softer, highly magnetostrictive material of the upper layer.
18. The sensor as claimed in claim 17, wherein lower layer
comprises a magnetic layer coupled to an anti-ferromagnetic
layer.
19. The sensor as claimed in claim 15, wherein the highly
magnetostrictive material of the upper layer includes an amorphous
alloy of Co.
20. The sensor as claimed in claim 15, further comprising a
mechanism for physically elongating or compressing said highly
magnetostrictive material of said upper layer.
21. The sensor as claimed in claim 15, further comprising a
measuring element coupled to said electrodes for measuring a
current flowing in said current path when stress is applied to said
highly magnetostrictive material of said upper layer, and, based on
the measured current, outputting a signal indicative of a change in
a physical dimension of said highly magnetostrictive material of
said upper layer, said change being caused by said stress.
22. The sensor as claimed in claim 15, wherein said highly
magnetostrictive material of said upper layer is elongatable by at
least 33%.
23. A sensor for measuring mechanical changes in length, comprising
a multilayer structure comprising a plurality of layers stacked one
upon another in a thickness direction of said structure; said
layers comprising upper and lower layers which are electrodes and
an intermediate layer sandwiched between said upper and lower
layers; said intermediate layer being a tunnel barrier which
exhibits the tunnel magneto-resistance (TMR) effect, and defines,
together with said electrodes, a tunnel current path that extends
from the upper layer, through an entire thickness of the
intermediate layer and to the lower layer; and wherein said upper
layer includes a highly magnetostrictive material that responds to
stress applied thereto; and said lower layer is a reference layer
having low magnetostriction or a magnetostriction sign opposite to
that of the highly magnetostrictive material of said upper
layer.
24. A method of measuring physical changes in length, said method
comprising providing a multilayer structure comprising a plurality
of layers stacked one upon another in a thickness direction of said
structure; wherein said layers comprise upper and lower layers
which are electrodes and an intermediate layer sandwiched between
said upper and lower layers; said intermediate layer is a tunnel
barrier which exhibits the tunnel magneto-resistance (TMR) effect,
and defines, together with said electrodes, a tunnel current path
that extends from the upper layer, through an entire thickness of
the intermediate layer and to the lower layer; and said upper layer
includes a highly magnetostrictive material that responds to stress
applied thereto, and the highly magnetostrictive layer includes a
CoFe alloy; applying stress to said highly magnetostrictive
material of said upper layer to cause a change in a physical
dimension of said material of said upper highly magnetostrictive
layer; measuring a current flowing in said current path when said
stress is applied to said highly magnetostrictive material of said
upper layer; and based on the measured current, outputting a signal
indicative of said change in the physical dimension of said
material of said upper highly magnetostrictive layer.
25. The sensor as claimed in claim 15, wherein said CoFe alloy
includes one selected from the group consisting of
Co.sub.50Fe.sub.50 and
(Fe.sub.90Co.sub.10).sub.78Si.sub.12B.sub.10.
26. The sensor as claimed in claim 15, wherein said lower layer is
a reference layer having low magnetostriction or a magnetostriction
sign opposite to that of the highly magnetostrictive material of
said upper layer.
Description
FIELD OF THE INVENTION
The present invention relates to a sensor for measuring mechanical
changes in length, in particular a mechanical pressure and/or
tension sensor, comprising a sandwich system with two flat,
superposed electrodes separated by a tunnel component (tunnel
barrier), in particular by an oxide barrier exhibiting tunnel
magneto-resistance (TMR).
BACKGROUND OF THE INVENTION
Magneto-resistance sensors that are based on the GMR (Giant
Magneto-Resistance) effect (P. Grunberg, R. Schreiber, Y. Pang, M.
B. Brodsky, H. Sowers, Physical Review Letters 57, 2442 [1986]:
"Layered Magnetic Structures: Evidence of antiferromagnetc coupling
of Fe-Layers across Cr-interlayers") illustratively are used as
angular sensors or read heads in hard disk drives. In general
however only the very high sensitivity to magnetic fields is being
exploited for such purposes. Attempts already have been made to use
so-called TMR (Tunnel Magneto-Resistance) elements as non-volatile
magnetic storage media (MRAM, magnetic random access memory). The
principles involves are summarily discussed below. A conventional,
voltage-insensitive GMR sensor is configured as follows:
In the simplest case, two magnetic layers illustratively cobalt
layers are separated by a non-magnetic layer, for instance made of
copper. At the proper spacer layer thickness, the magnetic layers
will couple anti-ferromagnetically as long as the external field is
zero. If an external magnetic field is applied, the direction of
magnetization of the magnetically softer layer will be rotated. At
saturation, the two magnetic layers will couple parallel to each
other. An electrical resistance differential arises between these
two states.
This relative change in resistance, which is caused by an angular
change of the directions of magnetization, is described by the
relation .DELTA.R/R=(.DELTA.R/2R).sub.max(1-cos .alpha.) where
(.DELTA.R/R).sub.max denotes the maximum relative change in
resistance of a given system of layers and where .alpha. denotes
the angle between the two directions of magnetization of the two
magnetic layers.
Moreover there are layer configurations for which the magnetic
layers remain uncoupled on account of a more substantial thickness
of the spacer layer. The lower layer consists of a hard-magnetic
material exhibiting uni-axial anisotropy pointing anti-parallel to
the orientation of the soft-magnetic layer. This condition is
typically attained using as the lower layer a magnetic layer
coupled to a synthetic anti ferro-magnet and a soft-magnetic layer
as the upper layer, said upper layer being rotatable by an external
magnetic field. In the case of GMR, so-called "spring valves" are
used. Relative resistivity changes .DELTA.R/R of 3% to a maximum of
5% have been measured at room temperature for such configurations.
Substantially higher values may be attained with multi-layer
systems.
Basically the TMR structures exhibit a similar behavior as the GMR
components. They are characterized in that the two magnetic
electrodes are separated by a thin oxide barrier instead of a
metallic, non-magnetic spacer layer. The tunnel current through the
barrier depends on the directions of the electrode magnetizations
as long as spin-flip dispersion is averted.
Isotropic ferro-magnets exhibit a magneto-elastic energy density
described by E.sub.me=-(3.sigma..lamda..sub.s cos.sup.2.theta.)/2
where .lamda..sub.s is the magnetostriction at saturation and where
.sigma. is the external mechanical stress. This energy density
describes the interaction between the magnetic torques and the
internal and external mechanical stresses. The angle between the
stress axis and the direction of magnetization is denoted by
.theta..
As regards positively magnetostrictive material under mechanical
tension, it follows that the torques align in the direction of the
axis of tension. Compressive stresses cause orientation
perpendicularly to the stress axis. This behavior is reversed for
negatively magnetostrictive materials.
The ratio of the magneto-elastic energy E.sub.me to the total
energy E is denoted by the magneto-mechanical coupling coefficient
k.sub.33. This coefficient is defined as follows:
k.sub.33=E.sub.me/E.sub.tot.
The elongation sensitivity GF=(.DELTA.R/R)/.DELTA..epsilon. [gauge
factor] i.e. the gains for the metal-based strain gauges are
between 2 and 4. The so-called piezo-resistive sensors based on
doped silicon are between 80 and 180.
Already a substantial number of magneto-resistance sensors using
magnetostrictive materials is known. Illustratively U.S. Pat. No.
5,168,760 discloses a magnetic multi-layer exhibiting a periodic
sequence of two different layers, one ferromagnetic, the other
non-ferromagnetic. The ferromagnetic layers always couple to each
other in anti-parallel manner. By applying a small magnetic field,
the anti-ferromagnetic coupling of the layer torques is slightly
changed toward ferromagnetic coupling. If magnetostrictive layers
are used as the ferromagnetic layers, then an external mechanical
stress may entail further rotation of the magnetic torques toward
ferromagnetic coupling, resulting in a large change in
resistance.
Moreover a two-element sensor based on the GMR effect is known
whereby the effect of mechanical stress and magnetic field on the
sensor signal may be separated. One hard-magnetic layer with a
given direction of magnetization is used in both sensor elements
and furthermore two soft-magnetic layers each time are separated by
a non-magnetic one. These soft-magnetic layers are exposed to an
oppositely directed magnetic biasing field of the same intensity.
As a result the above cited separation of sensor signals may be
attained by analyzing the sum and difference signal.
The following design also is known: a sensor consists of a pinned
magnetic layer, of a non-magnetic layer and a free magnetostrictive
layer in a configuration exhibiting the high magnetoresistive
effect. This design exploits the fact that the permeability of the
free magnetic layers changes on account of magnetostriction. When
an appropriate magnetic biasing field is applied, a mechanical
stress will entail a strong change in electrical resistance.
U.S. Pat. No. 5,856,617 describes a GMR layer configuration to
measure the deflection of an AFM (atomic force microscope)
cantilever beam. The magnetically free layer of this configuration
exhibits non-vanishing magnetostriction. This document discloses a
GMR layer configuration having a magnetostrictive structure
composed of a triple layer of Ni--Fe, Ni and Co, and further
applications as an AFM sensor.
Another publication has disclosed the effect of amorphous CoFeNiSiB
layers acting as the magnetically soft layer in TMR elements. This
alloy substance is a non-magnetostrictive alloy. This research led
to observed TMR effects of 12%.
SUMMARY OF THE INVENTION
Accordingly it is the objective of the present invention to create
a sensor that shall be used in particular to accurately and
precisely detect mechanical values. Such a sensor also shall allow
maximal miniaturization compared to known strain gauges or
magneto-elastic sensors.
The basic concept for this novel sensor is a special combination of
thin films exhibiting a TMR (tunnel magneto-resistance) effect with
layers that exhibit magneto-elasticity. The magneto-elastic
material used in the proposed sensor advantageously shall exhibit
as high as possible a magneto-mechanical coupling coefficient
k.sub.33 which is equivalent to high sensitivity. Magneto-elastic
sensors that are based on thin tapes of films attain sensitivities
up to 2.times.10.sup.5.
The invention offers the considerable advantage of high spatial
resolution of the new sensor. Compared with competing technologies
such as stratified composites with piezoelectric effects or the
above cited principles, TMR structures are characterized by their
small lateral dimensions. Accordingly they enable applications
requiring sensor arrays which elude the competing technologies, for
instance in bio sensor applications or regarding data storage using
AFM stylus arrays.
As regards the sensor of the present invention, the individual
magnetic layers of conventional TMR layer systems are replaced by
special layers exhibiting magnetostrictive properties. Highly
magnetostrictive materials must be used which furthermore exhibit
high spin polarization and accordingly, besides high sensitivity to
elongation, also enable sufficiently high .DELTA.R/R values of
about 20 to 50%. Alloys containing CoFe were found to be extremely
well suited. Consequently mechanical magnitudes may be measured on
account of a relative range of resistance in the TMR system taking
place on account of a magneto-elastically induced change of the
direction of the magnetic torques in those layers. Therefore the
sensitivity to elongation of our design should exceed that of
conventional metals or strain gauges and the semiconductor-based
sensors. It was found that substantially higher sensitivities or
magnifying factors could be attained for typical changes in
resistance of TMR elements and typical ranges of elongation when
reversing the magnetization of magnetostrictive layers. The
experimental work corroborated the theoretical estimates.
Moreover the TMR sensors allow significant improvements regarding
size and hence better spatial resolution in mechanical strain
gauges because these TMR elements may be manufactured is sizes of a
few 100 nanometers (nm). Interferences such as external magnetic
fields or temperature changes may be compensated by a special,
multilayer design, for instance by mounting the sensors in a bridge
configuration.
The applicability of such sensors in particular relates to
accurately and precisely determine mechanical magnitudes. On
account of the compact sensor structure attained with thin-film
technology, measurements also may be carried out on components or
structures of which the sizes are in the micron (.mu.) range. Using
the TMR sensors of the invention allows significant improvement as
regards sensitivity, compactness and hence spatial resolution of
the mechanical strain gauge. The new sensor offers substantial
improvement over the extant technologies in at least one
performance aspect (sensitivity or compactness).
Pertinent tests have shown that after the barrier has been
deposited, the vacuum may be interrupted and the mating electrode
may be deposited in another chamber without thereby entailing
considerable signal losses. Photolithographic structuring stages in
making the lower electric contacts are required for the TMR sensor
to set the tunnel current. Relative changes in resistance
.DELTA.R/R larger than 40% have been observed in TMR structures.
Furthermore TMR elements may be manufactured with dimensions in the
sub micron range.
BRIEF DESCRIPTION OF THE DRAWINGS
Illustrative embodiments of the present invention are elucidated
below in relation to FIGS. 1 through 3.
FIG. 1 shows a sensor layer sequence,
FIG. 2 shows the resistance as a function of the applied magnetic
field, and
FIG. 3 shows the effect of compressive stress on a
20.mu..times.20.mu. magnetic tunnel element fitted with an
amorphous [Fe.sub.90, Co.sub.10).sub.78Si.sub.12B.sub.10]
magnetostrictive layer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is an exploded view of a magneto-elastic TMR sensor used to
measure mechanical magnitudes. This sensor comprises a magnetically
hard layer 1 which is separated by a tunnel barrier 2 from a
magnetostrictive layer 3 which in particular may be an alloy
containing CoFe. By applying an external mechanical stress (arrow
.sigma.), the direction of magnetization (arrow A) changes in the
manner described above and thereby the resistance of the system
crossed by the current I. Experiments carried out on such sensors
with magnetically soft CoFe layers and amorphous (FeCo)SiB alloys
are described below.
The magnetic tunnel elements are made by sputtering. The
magnetically pinned layer consists in all samples of an 8 nm
Ir.sub.23Mn.sub.77 antiferroamgnetic (AF) layer and of a 2.5 nm
layer of Co--Fe. A 1.5 nm thick layer of aluminum is deposited as
the tunnel barrier and is oxidized by plasma oxidation.
Magnetostrictive Fe.sub.50Co.sub.50 and amorphous
(Fe.sub.90Co.sub.10).sub.78Si.sub.12B.sub.10 alloys nominally 6 nm
thick are used as the free layers.
The magnetic tunnel elements were sputtered with the
magnetostrictive soft-magnetic Fe.sub.50Co.sub.50 layer and the
magnetic tunnel elements with the
(Fe.sub.90Co.sub.10).sub.78Si.sub.12B.sub.10 soft-magnetic layer
was carried out after they were removed from the vacuum following
aluminum layer oxidation. The subsequent sputtering of the
soft-magnetic magnetostrictive layer was carried out four days
later in another procedure.
To ascertain the effect of the applied mechanical tension on
magnetic tunnel elements and on TMR-based strain gauges, a bending
device was built which allows measurements of magnetic-field
dependent resistance at up to 1.8 tesla and simultaneously
measurement of the homogeneous sample elongation. Homogeneous
elongation of the magnetic tunnel elements is attained using the
so-called four-point bending method. Elongation is implemented by
displacing a so-called slider containing two ceramic bars in
particular 3 mm in diameter. These bars are mutually 6 mm apart and
configured centrally between two fixed supports (3 mm dia.; 18 mm
apart).
FIG. 2a shows the effect of applied mechanical tension on a
20.mu..times.20.mu. magnetic tunnel element which was prepared
including a soft-magnetic Co.sub.50Fe.sub.50 layer 6 nm thick. The
minor-loop measurement shown in FIG. 2a is carried out in the
parallel configuration (the applied tension is parallel to the
magnetic orientation of the magnetic tunnel element and to the
applied magnetic field). The black, dashed curve of resistance vs
applied magnetic field represents the unstretched state of the
magnetic tunnel element whereas the dark-gray and the light-gray
curves each resp. represent measurements at 0.33% and 0.66%
elongation. The tunnel magneto-resistance is 20% and is nearly
constant for all three measurements. The reversal behavior of the
magnetostrictive Fe.sub.50Co.sub.50 soft-magnetic layer shows a
null point shift of 1.5 kA/m (19 oersteds) from the null field due
to the Neel coupling between the ferromagnetic layers (B. D. Schrag
et al, Appl. Phys. Lett., vol. 77, pp 2373, October 2000, "Neel
orange-peel coupling in magnetic tunnel junction devices"). This
behavior reveals a steeper rise, namely an increase in coercitive
field intensity and a shallower rise of the tunnel
magneto-resistance.
FIG. 2b shows the measurement of a similar magnetic tunnel element
in the so-called parallel configuration, though in this instance
under compressive stress. A decrease in slope and narrower
hysteresis of the soft-magnetic, magnetostrictive layer will be
observed. These changes are attributed to a stress-induced change
in the anisotropy of the soft-magnetic layer. A 50% change of the
initial tunnel magneto-resistance is expected from the maximally
possible change of 90.degree. of the direction of magnetization of
soft-magnetic layer due to the applied stress. Data analysis shows
this 50% change of the tunnel magneto-resistance also (17% to 8%)
for a relative change in elongation .DELTA..epsilon. of 1.1%.
On account of their high susceptibility to elongation, amorphous,
magnetostrictive Fe-based alloys are appropriate materials for the
strain gauges of the present invention. Accordingly
(Fe.sub.90Co10).sub.78Si.sub.12B.sub.10 was selected as the
material with which to develop this high-sensitivity
tunnel-magnetoresistive strain gauge.
FIG. 3 shows a typical measurement of a soft-magnetic, amorphous,
magnetostrictive (Fe.sub.90Co.sub.10).sub.78Si.sub.12B.sub.10 layer
using a magnetic tunnel element 20.mu..times.20.mu. in size. The
dashed black line (resistance vs applied magnetic field) shows the
unstressed state of the magnetic tunnel element, the dark gray and
light gray curves resp. having been measured 0.33% and. 55%
elongation. The tunnel magneto-resistance is 33% and is nearly
constant in all three curves. A 50% change in the initial tunnel
magneto-resistance (from 30% to 15%) is observed at
.DELTA..epsilon.=0.55%. The gain is 300 for the soft-magnetic
amorphous, magnetostrictive (Fe.sub.90
Co.sub.10).sub.78Si.sub.12B.sub.10 tunnel element. As regards TMR
sensors comprising magnetostrictive Fe.sub.50Co.sub.50 layers 2.5
nm thick and prepared entirely under vacuum, tunnel
magneto-resistances of 48% were measured. The gains were 450
600.
The design of a highly sensitive magnetostrictive sensor entails
the required and simultaneous optimization of a plurality of
properties. Illustratively a large magneto-resistance effect is
required, further a large magneto-elastic coupling coefficient for
the sensor layer, also a reference layer of low magnetostriction or
with a sign opposing magnetostriction, and a tunnel barrier that
remains undegraded by mechanical stresses. It was observed that a
conventional TMR structure might be used for angle sensors and
would show a signal of about 20%, on the other hand will not
respond to mechanical stresses. Moreover the
(Ni.sub.84Fe.sub.16/Cu/Co/FeMn) GMR structure cannot be directly
transferred to TMR structures because their insufficient spin
polarization would result in too low a signal and the NiFe alloy
used exhibits only low magnetostriction. As regards the
illustrative invention embodiments discussed herein, we were able
to simultaneously meet said partly conflicting requirements by
using crystalline or amorphous Co alloys of high spin polarization
and high magnetostriction and/or by selecting exchange-bias systems
wherein the pinned layers react only slightly or not at all to
external stresses.
Illustrations of the invention include sensors comprising a
reference layer free of magnetostriction and a measuring layer
which is magnetostrictive, the said reference layer exhibiting a
sign different from the measuring layer magnetostriction and the
reference and/or measuring layers consisting of several
ferromagnetically coupling layers. The measuring layer may consist
of magnetostrictive, crystalline or also amorphous alloys
containing Fe and Co. Moreover layers made of maximally
magnetostrictive materials such as rare earth materials, for
instance TERFENOL (Tb, Dy) Fe.sub.2 and combinations for instance
of the sort of CoFe(Tb, Dy)Fe.sub.2 are conceivable. The following
stacked layers are cited as illustrative embodiments:
Ta5/Cu30/IrMn8/CoFe2.5/Al.sub.2O.sub.3/CoFe2.5/Ta10
Ta5/Cu30/Ni6/Al.sub.2O.sub.3/CoFe6/Ta10
Ta5/Cu30/Ni6/CoFe1/AL.sub.2O.sub.3/CoFe6/Ta10
Ta5/Cu30/Ru5/IrMn8/CoFe2.5/Al.sub.2O.sub.3/FeCoSiB6/T10
Ta5/Cu30/Ru5/IrMn8/CoFe2.5/Al.sub.2O.sub.3/((Tb,Dy))Fe6/Ta10
Ta5/Cu30/Ni6/CoFe1/Al.sub.2O.sub.3/CoFe1/((Tb,Dy))Fe6/Ta10.
All numbers in the stacked layers are in nm. Moreover
CoFe.dbd.Co.sub.50Fe.sub.50;
FeCoSiB.dbd.(Fe.sub.90Co.sub.10).sub.78Si.sub.12B.sub.10; the
thickness of Al.sub.2O.sub.3 is 2 nm.
* * * * *